The present invention relates to a liquid crystal display device in general, and in particular to a display device with reduced unevenness of display. While liquid crystal display devices have taken many forms simple matrix type liquid crystal display devices are generally driven by a voltage averaging method. The liquid crystal panel, is provided with scanning and signal electrodes each having a resistance which is greater than zero (0) and a liquid crystal layer which acts as a dielectric. Therefore, the effective voltages at the display elements or dots formed by the intersection of each scanning electrode and signal electrode changes depending on the nature of the characters and the images displayed by the liquid crystal panel. This results in unevenness on the display device.
The problem described above has been known in the art. Many problem solving techniques have been used in the past, such as, a method of reversing the polarity of the voltage applied to the liquid crystal panel a plurality of times in one frame (hereinafter referred to as "line reverse driving method"). This method is described in Japanese Patent Application Laid-Opened Official Gazette No. 62-31825, No. 60-19195 and No. 60-19196.
Further, other methods for improving unevenness of display are known in the art such as the method described in Japanese Patent Application No. 63-159914 proposed by the present inventor, referred to as the "voltage correcting method".
The line reverse driving method is an effective method of improving unevenness of display caused by variations of the optical characteristics of the liquid crystal caused by changes in the frequency component of the applied voltage. The line reverse driving method is effective at improving the unevenness of display, but cannot completely remove the unevenness of display caused by changes in the frequency component of the applied voltage.
The unevenness of display can be improved by the voltage correcting method proposed by the present inventor. However, with the application of this method the unevenness of display such as described below has not been eliminated.
Referring to FIG. 1 the unevenness of display remaining after application of the voltage adjusting method is explained. FIG. 1 depicts a liquid crystal panel generally indicated as 1, composed of a liquid crystal layer 5, a first substrate 2 and a second substrate for sandwiching the liquid crystal layer 5 therebetween. A plurality of scanning electrodes Y1 through Y6 are formed on substrate 2 in the horizontal direction and a plurality of signal electrodes X1 through X6 are formed on substrate 3 in substantially the vertical direction. Each intersection of scanning electrodes Y1 through Y6 and signal electrodes X1 through X6 forms a display element (dot) 7. Display elements 7 marked with crosshatching represent the lighting or illuminated state and blank display elements 7 represent the non-lighting or non-illuminated state. Further, FIG. 1 is depicted as a checkered pattern or matrix. The display panel of FIG. 1 is limited to a 6.times.6 matrix or 36 display elements for simplicity, however, in exemplary embodiments the number of display elements of liquid crystal panel 1 may be much greater.
In the voltage adjusting method, a scanning voltage correcting wave is applied to the scanning electrodes. For example, a scanning voltage correcting wave is applied to the left side of the scanning electrode, to vary the display pattern. Among the examples of the voltage correcting method of driving liquid crystal displays proposed by the present inventor in Japanese Patent Application No. 63-159914, the scanning voltage correcting wave is applied to every other line display, thereby, improving the unevenness of the display. Specifically, the correcting voltage is superimposed upon the non-selective voltage in accordance with the difference I between the number of lighting display elements on a scanning electrode and the number of lighting display elements on the following scanning electrode when the selection is moved from one scanning electrode to the next scanning electrode. However, in the case of the display subject shown in this figure, since the difference I is always zero (0), the correcting voltage is not applied to the non-selective voltage.
The signal voltage wave is fed alternatively to the signal electrodes from the upper and the lower ends of the signal electrodes, with each consecutive signal electrode receiving the signal voltage wave from the opposite direction. The liquid crystal panel 1 displays a "positive display" which becomes dark when the effective voltage applied to the display dot becomes higher.
When the display patter depicted in this figure is actually used, the display dots formed by electrodes X1, X3 and X5 are brighter on the upper portion, and become darker on the lower portion. On the contrary, the display dots formed by electrodes X2, X4 and X6 are brighter on the lower portion, and become darker on the upper portion. In other words, the effective voltage actually applied to the display dot is greatest in the dot most proximate to the source of the signal voltage wave, and the effective voltage decreases as the display dot increases in distance away from the signal voltage wave.
The following problems have been discovered through further experimentation on the unevenness of display. Particular reference is made to FIGS. 2(a) through (c) in order to explain the problems identified.
FIGS. 2(a) through (c) depict examples of actual driving waveforms (waveforms of applied voltage) applied to the electrodes of the liquid crystal panel shown in FIG. 1. In FIG. 2(a), the full line shows the voltage waveform on the signal electrode X3 in the position of display element D31 (the intersection of X3 and Y1) of FIG. 1. The dotted line shows the voltage waveform on the signal electrode X2 in the position of the display dot D21 (the intersection of X2 and Y1). The wave depicted with a full line and the wave depicted with a dotted line are drawn to be slightly shifted to distinguish them from each other and to facilitate the explanation. The two waveforms are actually superposed upon each other.
FIG. 2(b) shows the voltage waveform on signal electrode X1 in the position of display dot D21 or D31 in FIG. 1. FIG. 2(c) shows the difference between the voltage wave on scanning electrode Y1 and the voltage wave on signal electrode X3 in the position of display dot D31 in FIG. 1. The full line depicts the voltage wave applied to the display dot D31. Similarly, the dotted line in FIG. 2(c) depicts the voltage wave applied to display dot D21. The portion with hatching shows the difference in applied voltage between the lighting dot and the non-lighting dot, which is not the voltage difference causing the unevenness of display.
In the figure V0, V1, V2, V3, V4 and V5 represent the applied voltages. The selective and non-selective voltages are applied to the scanning electrode and the lighting and the non-lighting voltages are applied to the signal electrodes. The voltages V5, V3, V0 and V4 are defined as the first group of lighting, non-lighting, selective and non-selective voltages and the voltages V0, V2, V5 and V1 are defined as the second group of lighting, non-lighting, selective and non-selective voltages (hereinafter, the voltage wave applied to the scanning electrode is referred to as scanning voltage wave and the voltage wave applied to the signal electrode is referred to as the signal voltage wave). The first and second voltage groups are periodically switched. In this example, the voltage groups are switched after all the scanning electrodes Y1 through Y6 are applied with the selective voltage (this cycle is known as one frame, and it is represented by F1 and F2 in FIG. 2).
As shown in FIG. 2(a), since the distance between the display element D31 and the end portion applied with the signal voltage wave is short, the damping of the voltage wave is almost nonexistent and the applied signal voltage wave is applied as is without any rounding or damping. However, as shown in FIG. 2(b), since the distance between the dot D21 and the end portion applied with the signal voltage wave (hereinafter referred to as the "driving end") is large in the case of signal electrode X2 in the position of the dot D21, the result is a signal voltage wave having large damping and rounding.
In other words, the damping and rounding of the voltage wave is caused by an integrating circuit, which includes the electrical resistance internally within signal electrodes X1 through X6 and the condenser having the liquid crystal material as the dielectric. Therefore, when signal electrodes X1, X3 and X5 are changed from the lighting to non-lighting voltage and from the nonlighting to lighting voltage, a larger spike type noise is generated than when signal electrodes X2, X4 and X6 are changed from the lighting to non-lighting voltage and from the non-lighting to lighting voltage, when considering scanning electrode Y1. The spike type noise generated on scanning electrode Y1 by switching signal electrodes X1, X3 and X5 from the lighting to non-lighting voltage and from the non-lighting to lighting voltage, thereby, dominates. Therefore, as shown by the full line waveform shown in FIG. 2(c), the effective voltage applied to display element D31 decreases, and as shown by the dotted line, the effective voltage applied to display dot D21 increases.
Alternatively, in the case of scanning electrode Y6 shown in FIG. 14 the noise generated by the signal electrodes X2, X4 and X6 dominate. Further, the effective voltage applied to the display dot D26 decreases and the effective voltage applied to the display dot D3 increases.
Hereinafter we will refer to the mth signal electrode from the left side as signal electrode Xm, the nth scanning electrode from the upper portion of liquid crystal as scanning electrode Yn, and the display dot formed at the intersection of signal electrode Xm and scanning electrode Yn will be referred to as display element Dmn. Generally, when the selection of successive scanning electrodes moves from scanning electrode Yn to scanning electrode Yn+1, and the signal voltage wave applied to those signal electrodes receiving the signal voltage wave from the end portion depicted at the upper portion of FIG. 14, is such that the lighting voltage is successively applied when scanning electrodes Yn and Yn+1 are scanned (successive lighting display elements), the case is defined as a1. When the non-lighting voltage is successively applied when scanning electrodes Yn and Yn+1 are scanned (successive non-lighting display elements), the case is defined as b1. When the lighting voltage is applied to the display element formed on scanning electrodes Yn and the non-lighting voltage is applied to the scanning electrode Yn+1, the case is defined as cl. When the nonlighting voltage is applied to the display element formed on scanning electrodes Yn and the lighting voltage is applied to the scanning electrode Yn+1, the case is defined as d1.
Similarly, when the selection of successive scanning electrodes moves from scanning electrode Yn to scanning electrode Yn+1, and the signal voltage wave applied to those signal electrodes receiving the signal voltage wave from the end portion depicted at the lower portion of FIG. 14, is such that the lighting voltage is successively applied when scanning electrodes Yn and Yn+1 are scanned (successive lighting display elements), the case is defined as a2. When the non-lighting voltage is successively applied when scanning electrodes Yn and Yn+1 are scanned (successive non-lighting display elements), the case is defined as b2. When the lighting voltage is applied to the display element formed on scanning electrodes Yn and the non-lighting voltage is applied to the scanning electrode Yn+1, the case is defined as c2. When the nonlighting voltage is applied to the display element formed on scanning electrodes Yn and lighting voltage is applied to the scanning electrode Yn+1, the case is defined as d2.
The number of lighting display elements 7 on scanning electrode Yn that are applied with the signal voltage wave from one end portion of the display (the upper portion of FIG. 14) is N1.sub.ON and the number of non-lighting display elements 7 is N1.sub.OFF. Further, the number of lighting display elements 7 on scanning electrode Yn+1 that are applied with the signal voltage wave from one end portion (the upper portion of FIG. 14) is M1.sub.ON and the number of non-lighting display elements 7 is M1.sub.OFF. Similarly the number of lighting display elements 7 on scanning electrode Yn that are applied with the signal voltage waveform from one end portion (the lower portion of FIG. 14) is N2.sub.ON and the number of non-lighting display elements 7 is N2.sub.OFF. Further, the number of lighting display elements 7 on scanning electrode Yn+1 that are applied with the signal voltage wave from one end portion (the lower portion of FIG. 14) is M2.sub.ON and the number of non-lighting display elements 7 is M2.sub.OFF. The relationship between scanning electrodes and signal electrodes that are applied with signal voltage waves from one end portion (the upper portion of FIG. 14) is as follows: EQU N1.sub.ON =a1+c1 EQU N1.sub.OFF =b1+d1 EQU M1.sub.ON =a1+d1 EQU M1.sub.OFF =b1+c1
Herein, the numeric value I1 is defined as follows: ##EQU1## Similarly, the relationship between scanning electrodes and signal electrodes that are applied with signal voltage waves from one end portion (the lower portion of FIG. 14) is as follows: EQU N2.sub.ON =a2+c2 EQU N2.sub.OFF =b2+d2 EQU M2.sub.ON =a2+d2 EQU M2.sub.OFF =b2+c2
Herein, the numeric value I2 is defined as follows: ##EQU2##
Further, the function I(k) is defined as follows: EQU I(k)=f(k).multidot.I1+f(L-k).multidot.I2.
The function f(k) is a function which decreases as k increases. The function f(k) shows that the spike type noise generated in the scanning electrode, by each signal electrode, increases as the signal voltage wave approaches the driving end (the end where the signal voltage wave is applied).
The character L indicates the total number of scanning electrodes. The relationship between k and L is as follows: EQU 1.ltoreq.k.ltoreq.L.
The absolute value of the function I(k) defines the spike type noise generated on the kth scanning electrode Yk when the selection is moved from the scanning electrode Yn to scanning electrode Yn+1. Thus, the function I(k) increase as the noise generated decreases. The direction in which the noise is generated dependent upon whether the value of the function I(k) is positive or negative.
In scanning electrode Yk, if the direction of the voltage spike type noise according to the function I(k) is in phase with the variation of the voltage wave applied to each signal electrode, then the effective voltage applied to the display element formed with the signal electrode and the scanning electrode Yk becomes lower, thereby, making the display element brighter. Alternatively, if the phases of the voltage spike I(k) and the signal electrode are reversed, then the effective voltage applied to the display element will be great, thereby, making the display element darker. Thus, unevenness of display would remain. The different types of unevenness remaining after the voltage correcting method will be explained with reference to FIGS. and 4.
Particular reference is made to FIGS. 3 and 4 , wherein liquid crystal panels with different display subjects are depicted. FIGS. 3 and 4 depict the same liquid crystal elements as FIG. 1. Thus, similar numbers are used to designate similar elements of the liquid crystal panel. The voltage correcting method of decreasing unevenness of display as described in Japanese Patent Application No. 63/159914 proposed by the present inventor describes applying a varied scanning voltage to the left side of the scanning electrodes Y1 through Y6 according to the pattern of the display elements 7 upon the display. Accordingly, the scanning voltage wave is varied by superimposing the correcting voltage upon the selective voltage in accordance with the number of lighting dots Z on the scanning electrode selected. In FIGS. 3 and 4 the congruence quadrangle is displayed in the position shifted to the left end and the right end respectively. Therefore, the same correcting voltage is applied to each scanning electrode Y1 through Y6 of the liquid crystal panel 1 when it is in a condition displaying either FIG. 3 or FIG. 4.
In FIG. 3 there is unevenness of display generated in the display quadrangle that is manifested in the form of horizontal darkening resulting from excess correcting voltage. On the contrary, in FIG. 4, the correcting voltage is not sufficient to cause unevenness by weft pulling. In FIG. 4 horizontal brightening remains because the condenser formed with the resistance of each scanning electrode Y1 through Y6 forming a part of the liquid crystal panel 1 forms an integral circuit and the lighting display element forms a condenser having larger capacitance as compared with the non-lighting dot. The interference causing rounding of the voltage wave of the lighting display element at a greater distance from the end of the scanning electrode applied with the scanning voltage wave (hereinafter the "driving end") is larger than the interference of the non-lighting display element at about the same position. Thus, larger rounding results in display elements that are more distanced from the driving end of the scanning voltage wave. Thus, if the lighting dot exists in a position distanced from the driving end of the scanning electrode Y1 through Y6, the scanning electrode wave including the correcting voltage is rounded. Therefore, the effective voltage applied to the display dot decreases.
In a matrix with s scanning electrodes Y1 through Ys, the numerical value z' representing the unevenness of display by weft pulling, may be calculated by the following formula: ##EQU3## wherein, i designates the signal electrode upon which the display dot is turned on, for example, X.sub.i (i=1, 2, 3, . . ., P) designates the number of signal electrodes X1 through Xp.
Herein, the letters s and p designate the number of scanning electrodes and signal electrodes respectively.
The function q(i) is a function that increases as the value i increases.
The function .delta.(i) is 1 when the display element positioned in i on the selected scanning electrode is lighting, and it is 0 when the display dot positioned in i is non-lighting.
Therefore the numerical value z' increases greatly on display element most distanced from the driving end of the scanning electrode.
Thus, the unevenness of display has not been completely removed by the voltage correcting method of removing unevenness of display by the weft pulling of conventional displays. The numerical value z may be obtained by the following formula: ##EQU4## The unevenness resulting in the mechanisms described above have caused a decrease in the quality of the display. By this invention applicant seeks to improve the evenness of display.